Brandis tells High Court to take colleagues' citizenship vows in good faith

Dual citizen politicians who "honestly swear" they didn't know they held foreign citizenship should not be ruled ineligible to sit in Parliament, Attorney-General George Brandis has told the High Court.

In a submission concerning the fate of MPs currently under a citizenship cloud, Senator Brandis said his construction of the law suggested politicians should be taken at their word unless there was evidence to the contrary.

He argued it was not necessary to probe the citizenship status of MPs under respective foreign laws "in cases where a parliamentarian can honestly swear to being unaware" they were a foreign citizen or that there was a serious prospect that they were a foreign citizen.

"If there is material that contradicts such a claim, then the matter can be tested, as occurred in relation to Senator [Malcolm] Roberts," Senator Brandis wrote in a submission filed by Solicitor-General Stephen Donaghue.

"Otherwise, the oath of the parliamentarian should be accepted, and no question of disqualification would arise provided that reasonable steps were taken to renounce foreign citizenship within a reasonable time of the parliamentarian becoming aware of his or her status."

The Turnbull government is defending its ministers Barnaby Joyce, Fiona Nash and Matt Canavan, along with Senator Nick Xenophon and former Greens senator Larissa Waters, who is arguing that she is disqualified.

Senator Xenophon on Friday shocked the political world by announcing he would quit federal politics regardless of the court's ruling and instead seek election to the South Australian parliament.

Senator Brandis' argument hinges on the fact the MPs concerned were born in Australia, unlike Senator Roberts and former Greens senator Scott Ludlam, and had no reason to believe they were entitled to citizenship of another country.

"Without any knowledge of present or past citizenship there can be no risk of split allegiance," he argued. "In the context of that actual knowledge, the choice not to take all reasonable steps to renounce that citizenship marks its retention as voluntary."

The arguments will be thrashed out in the High Court, sitting as the Court of Disputed Returns, over three days of hearings this week.

Last week, former independent MP Tony Windsor - who has joined the case against his political nemesis Mr Joyce - argued allowing the Deputy Prime Minister to remain in Parliament would reward carelessness.

Mr Windsor's lawyers, including former solicitor-general Justin Gleeson, said letting the Nationals leader off the hook would encourage future candidates not to properly investigate whether they were dual citizens, because they would face no consequence for being ignorant.